Wireless local area networks use radio frequency (RF) transmissions to communicate between roaming computer devices and access points (or base stations). In a wireless networking environment, many computer devices may need to communicate within a given area. The RF transmissions associated with the devices share the same electromagnetic spectrum, and typically follow some protocol to avoid interfering with each other. Several sharing techniques are available to promote efficient usage of the spectrum. For example, the spectrum can be shared using time division multiplexing (TDM) and/or frequency division multiplexing (FDM).
At least one common FDM approach for sharing spectrum pre-allocates the spectrum into multiple channels, each channel being allocated a respective channel bandwidth. Even having multiple channels, however, further sharing is often needed to accommodate a large number of users. A contention-based scheme can be applied to each of the channels allowing multiple devices to operate on the same channel, one device at a time.
In a contention-based system, the devices typically have some means of determining when a channel is in-use, and therefore unavailable. For example, a receiver may sample the channel, asserting a “busy” signal responsive to the channel being unavailable. The device simply checks the busy signal prior to transmission and either continues with the transmission if the channel is available, or waits and tries again at a later time. Without such a means for determining channel availability, multiple devices may attempts to use the same channel at the same time, resulting in unwanted interference.
Even with the above precautions, it is still possible for communications equipment configured to listen to a single channel within a multi-channel communications environment to suffer from the following interference. Namely, the problem results from spectral leakage of energy from signals on adjacent channels. Communications equipment is designed to transmit energy within an intended channel bandwidth. However, the signal typically includes at least some amount of unwanted energy extending beyond the channel bandwidth. This unwanted energy is referred to as spectral leakage. Such spectral leakage can lead to equipment falsely concluding that a channel is busy (i.e., there is valid energy on its channel of interest), when in fact the channel is only occupied by the unwanted spectral leakage. In contention-based systems, the equipment reacts by unnecessarily relinquishing the channel, leading to a loss in throughput.